1. Field of the Invention
This invention relates to electromechanical transducer devices and particularly to piezoelectric shakers for calibrating accelerometers.
2. Description of the Prior Art
Piezoelectric accelerometers dynamically sense harmonic and non-harmonic acceleration, random acceleration and shock acceleration. When calibrated, accelerometers can electrically measure whatever acceleration they sense. Calibration establishes accelerometer sensitivity in terms of electrical output as a function of input force. Accelerometer sensitivity remains consistent up to about 5 kHz, but above this frequency, sensitivity increases with frequency increases. This rate of change of sensitivity, which directly affects the electrical output, is unpredictable, and as a consequence, high frequency dynamic calibration is necessary. Such calibration requires a device to provide sinusoidal motion with low distortion over a wide range of frequencies as the input force. Piezoelectric shakers, which convert controlled electrical input into sinusoidal motion, are well suited for this function.
Absolute calibration can be achieved by using the interferometric fringe disappearance method to measure absolute displacements based on the wave length of light. Comparison calibration is performed by comparing the output of a certified accelerometer, generally one that has undergone absolute calibration, against the output of the accelerometer being calibrated. This has been a two-step process that has prevented accelerometer calibration at the test site, and has additionally resulted in rather severe time and cost penalties inasmuch as the comparison requires that both the standard accelerometer and the one to be comparison calibrated against it must be set up separately on the shaker head.